ARCTIC SEA TO THE WEST OF GREENLAND. 61 



the ray the adambulacral plates stand next to the ventro-marginal plates, and are not 

 separated from them by the trapezoid imbricating scales above described. 



Each pair of mouth-plates forms an ovoid mass, the inner or apposed margins of 

 the plates being elevated into a prominent keel., The innermost pair of mouth-papillae 

 are very large and thick, and taper to a point the remainder, from 7 to 9 in number, 

 being considerably smaller and arranged round the free margin of the plate. Along, 

 or near to, the median keel of the mouth-plate are 3-5 coarse spinelets, the innermost 

 being large and thick, and are much less pointed than the marginal series. The 

 madreporiform body is frequently not more than its own diameter distant from the 

 margin, and is generally oval in outline and covered with elongate striae running in 

 the direction of the major diameter. 



The entire body and all its appendages are covered with an investing leathery 

 skin. 



Size. Ordinary specimens are about 30-40 millims. in diameter, the largest 

 recorded by Sars, from Tromso, being 65 millims. 



Colour. The colour is recorded as brown-red ; specimens preserved in spirit are 

 either black, greenish, or various shades of drab. 



Habitat. Ctenodiscus corniculatus is found in mud or soft clay bottoms at very 

 various depths, being dredged by Sars, at Finmark, in 40-200 fathoms depth ; and 

 further south, at Christiansund, in 40-80 fms. It has also been taken in 25 fms. at 

 Arksut, by Barrett ; and at Igaliko, in 60 fms., by Insp. Moller (Lutkeri). 



Premature Form. The young form of this species was described by Miiller and 

 Troschel under the name of Ct. pgmceus. Small individuals, of about half an inch in 

 diameter, are characterized by the natter test, the comparatively greater prominence of 

 the latero-dorsal spinelets, and the three large, conspicuously-developed spinelets which 

 are present on the terminal plate of the ray. The upper margin of this plate, which 

 lies towards the centre of the disk, is fringed with a series of papilla? similar to those 

 on the sides of the lateral plates ; and these papillae, as well as the spinelets, appear 

 to be subject to a greater or lesser degree of obliteration (or resorption) during the 

 progress of the growth of the Starfish ; in fact in old specimens the spinelets become 

 reduced to mere tubercles. 



According to Liitken, the apical prominence in the centre of the disk is more pro- 

 minent and characteristically developed in young forms ; but in the specimens which we 

 have examined (from Novaya Zemlya) it would appear to be quite the reverse, for we 

 have been unable to detect any difference, except a proportional diminution in size, 

 from the condition presented by the mature animal. The adambulacral plates in these 

 specimens bear their papillae or " ambulacral spines " on the furrow-margin, with one 

 large one placed thumb-like behind them on the aboral side, 



At the extremity of the ray there are only two of the ambulacral spinelets ; and 

 the thumb-like spinelet is larger than either of them, and is persistently present on every 

 plate. The mouth-plates are small and simple, having only three or four mouth- 

 papillae on the margin of each plate, and two only on the median ridge (or at most 

 three), the innermost of these being very large and prominent, and standing perpen- 



